1. Field of the Invention
This invention relates to an advanced cargo handling system for offshore discharge of containerships. It relates to a facility involving berthing subsystems. Multiple ships and lighters may be serviced at the same facility at the same time. The invention has particular applicability and advantage in heavier seas than can ordinarily be accomodated, for example, in sea state 3 conditions or higher and in a logistics over the shore (LOTS) environment.
2. Prior Art
The offloading of containerized and breakbulk cargo from large oceangoing ships using existing equipment is hazardous if not nearly impossible during periods of rough weather and high seas. Conventional cargo handling systems are not designed to cope with the relative motion between ships and lighters under such conditions. The U.S. Army has a stated need to stabilize the offloading interface between ships and lighters under sea states equivalent to SS3 and higher.
The cargo handling technology for LOTS operation available now and projected through the early 1990s is based upon hardware that was essentially known or available in the 1960s. In fact, the methods used today are little different from those employed in World War II. This limits operations to relatively calm seas. The LOTS technology presently available will not meet demands for operations in adverse weather now or in the future. System upgrades are required.
Using conventional or slightly modified cargo handling booms or cranes, cargo transfer to landing craft in SS3 or higher conditions becomes extremely hazardous for personnel, cargo, and transfer equipment. While load pendulation and vertical descent rates can be nominally controlled, heavy seas create significant motion on the lighter in six degrees of freedom (DOF). In SS3 or higher, present methods and resources used for safe control of the lighterage-cargo interface exceed human capabilities.
Commercial shipping companies, recognizing the economic advantages of employing state-of-the-art technology, are currently developing system solutions that employ automated robotics techniques. Such innovations include the Sea Land grid rail system and Matson's Bridge Crane installation. These companies recognize that future shipping programs will dictate much greater automation, higher productivity, improved control, and better system reliability.
In contrast to the systems approach of the commercial shipping companies, the traditional military approach has been to design and add-on more hardware to adapt less-capable equipment, such as cranes, for operations in heavy seas. This approach has not provided a totally acceptable solution.
The relative motion problem between ships and lighters in a LOTS environment is a subset of the entire offshore cargo handling problem. The ship-lighterage interface requirement cannot be divorced from any of the other system requirements. Thus, solving the interface problems while still using obsolete and unsatisfactory crane technology would not have a major beneficial effect on the overall offshore cargo handling problem.
It has previously been considered that operations in SS3 and higher could not be accomplished, primarily because of the lighterage interfaces at the ship and beach. The following are some of the conclusions amplifying this problem as derived from the JLOTS II Throughput Test Report: During the test, 14.5 days (46 percent) of JLOTS II were lost due to conditions equal to or greater than SS3. Delays were experienced on the auxiliary crane ship (T-ACS) used for cargo transfer between the containership and lighters with as little as one degree of roll. Measures to control pendulation (but without vertical motion compensation) resulted in "an overly complex, massive crane hook assembly, rider block, and automatic spreaders (that) significantly increased the difficulty in controlling load pendulation." The four-month training program for Able Bodied Seamen (ABS) did not prove sufficient for the highly sophisticated T-ACS cranes. Experienced marine crane operators are recommended for manning the system.
The T-ACS represents a considerable step forward in terms of past capabilities to meet system deployment and cargo throughput requirements. However, the T-ACS still shares the common problem of all prior systems in that the lighterage interface remains highly subject to sea state motion. Developmental efforts have been made to mitigate problems with motion and load control which have greatly added to the complexity and cost of cranes. These efforts have shown some promise, in that cranes can be made to work in adverse conditions but at considerable cost to control systems, training, and personnel requirements. While improvements could be seen in some SS3 cargo tests, crane system add-ons have contributed nothing to improved routine operations, that is, operations when seas are calm.
The Chief of Naval Operations concluded: "The finding from this experience is that a sustainable Sea State 3 capability does not exist. The primary limiting element is the lighterage." "Neither the T-ACS, Navy lighterage, nor beach facilities demonstrated an adequate capability to operate in SS3 conditions." "The TCDF (Temporary Container Discharge Facility, i.e., a 250-ton lifting capacity truck-crane mounted on a B Delong barge moored alongside the containership) is very susceptible to wave and swell action. Pendulation problems restrict its usefulness to calm weather operations."
Aside from significant difficulties with pendulation and load control, existing systems have a number of related problems that also constitute part of the difficulty and contribute to the hazard of offshore cargo transfer.
Among these problems are response intervals between when a load should be moved and when the crane operator actually moves it in response to a condition observed. Rarely can a crane operator see the load when he both acquires, lifts and deposits it. Generally, the operator's responses at the controls will be a product of his judgment, training, and experience; however, his actions will also reflect someone else's experience and background, specifically his signalman or hatch captain who may be directing him. Tagline handlers will also influence the cycle by their actions. Thus, a number of independent variables become part of the equation for moving cargo or loading a lighter. Fundamentally, these include crew communication, crew experience, and timeliness in responding to the environment. There exists no single, instantaneous, direct feedback to the crane's control system reporting changes in load position or control of where it should go. Only in calm seas where there is inconsequential motion and more time to react does the existing load control system work satisfactorily.
All loads are suspended by cables at the end of long booms. This is the single, most important element in preventing cargo from being effectively manipulated and controlled in SS3 conditions.
Cables and winches provided the fastest, simplest, and best means to maneuver cargo. However, loads suspended by cables depend upon gravity for control and have little means to influence centrifugal or other forces introduced in a seaway, except by some tagline modification. As a result, the attachment of spreader bars to contains is slow and difficult, often constituting 50 percent or more of the cycle time per container, even in calm seas. Positioning of containers on the deck of the lighter is a hit-or-miss proposition. Tipping moments can be created by picking heavy loads at extended distances during roll periods, and manpower requirements for taglines are excessive in relation to any assistance rendered in facilitating transfer. Meaningful control is not attained.
Until now, solutions to the lighterage interface problem have focused on devices and new hardware attached to the hooks of various cranes. However, as noted by the JLOTS II Joint Test Director (above), the T-ACS crane (to use one example) has become massive, complex, and unwieldy while still not able to fully solve the interface problem.
Representive patents dealing in general with this area include U.S. Pat. Nos. 4,795,298; 4,395,178; 4,180,362; 4,393,906; 4,317,524; 4,172,685; and 4,158,416. Many other patents in the field also exist but are not believed to suggest the present inventive concept.